Metal step coverage has been of prime importance throughout the history of integrated circuit (IC) manufacture. Step coverage, however, has still been a major problem for IC manufacturers even into the late 1980s. Poor step coverage can be found at the sharp vertical step metal to substrate contacts, metal to metal vias, and metal crossovers. As dimensions shrink, conventional techniques of producing sloped contacts and vias to improve step coverage fall short of expectations and are limited to stringent design criteria.
One method of improving step coverage has been realized through the use of thermal processing. Contact openings are cut using known dry etching processes. The sharp corners of the contacts resulting from the contact etch are rounded off by thermal processing at high temperatures. With the effect of surface tension, at temperatures close to melting point temperatures, almost all of the known insulating materials through which contact openings are formed tend to bead up eliminating sharp corners and reducing surface area. This reduction in surface area with no reduction in volume is a more thermodynamically stable arrangement.
The direct application, however, of thermal processing has certain disadvantages. For example, in the case of a metal contact, the contact resistance between metal and substrate is often reduced by the use of a suitable implant in the substrate. Thermal processing tends to nullify the effects of an implant because at melting point temperatures of reflow glass, the implanted species tend to diffuse out of the substrate and into the atmosphere before the metal layer is deposited. When such diffusion in the atmosphere occurs, a number of special implantation steps, according to the specific type of technology employed, may have to be added after thermal processing to achieve the desired contact resistance between metal and substrate.
It would be desirable for a semiconductor process to minimize step coverage problems in contacts and vias. It would further be desirable for such fabrication technique to provide sloped contact corners suitable for use with small device geometries.